Amarillo Travel and Info Center across I-40 from the Flying J truck stop is a good spot to stop/boondock for the night. Secure, bathrooms, vending machines, and wifi. Haven't been there in about 6 years since we discovered it, but it gets good reviews.
Amarillo/Texas boondockingThere's also Winslow, AZ (think Eagles song "Take It Easy", might be worth a quick look, if you like that sort of thing). They have a Walmart for free overnighting.
and Meteor Crater, a little further west along I-40 (again if you like that sort of thing). We got there late in the day the last time through, and thought what the heck, let's go see what it's about. They were just closing up. Asked if they'd let us take a quick look for an "after 4 pm" price, but they wanted the full entry fee which was a bit pricey to look at a big hole in the ground, so we just kept going. Maybe some other time.
Flagstaff has 2 Walmarts for low cost overnighting and restocking supplies.
Just north of Williams, off I-40, there's a nice campground, "Kaibab Lake CG". It is right off of AZ-64 which is one of the access routes north to the South Rim of the Grand Canyon. Very nice spot.
Kaibab Lake CG
Mike. Comments are anecdotal or personal opinions, and worth what you paid for them.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 Wrangler JKU (sold @ ????)
2016 Sunstar 26HE, V10, 3V, 6 Speed (sold @ 4600 miles)
2002 Roadtrek C190P (sold @ 315,000kms)